Field of the Invention
The present invention relates to an inertial sensor with vibrating masses, such as an angular sensor of the rate-gyro or free-gyro types.
Brief Discussion of the Related Art
Such an inertial sensor comprises a frame and seismic bodies (currently called seismic masses or test masses) which are generally positioned side by side and connected to the frame by means of elastic hinges so as to be movable in a suspension plane defined by two orthogonal directions of displacement of the seismic bodies. The elasticity of the hinges associated with the mass of the seismic bodies defines the frequencies of the own modes of the seismic bodies. The sensor also comprises actuators so arranged as to vibrate the seismic bodies and detectors so arranged as to detect movements relative to the seismic bodies.
The resonator composed of the seismic bodies and the elastic hinges has two vibration useful modes, which define two directions for the displacement of the seismic bodies. The detection of the vibration of the seismic bodies along such directions makes it possible to make an angular measurement.
The performances of such sensors are all the higher since the damping anisotropy of the resonator is stable. Such stability depends on the stability of the damping characteristics of the resonator and on the stability of the power transfers between the resonator and the outside. The damping anisotropy must be as stable as possible not to generate inaccuracy in the measurements made using the angular sensor.
Power transfers between the resonator and the outside depend on:                the average damping (in connection with the time constant of the sensor) which must be as low as possible, in order to limit the power input required for maintaining a constant amplitude of the vibration and limit the inaccuracy of such power input,        the balancing of the useful modes of vibration of the seismic bodies which must limit the reaction forces and torques on the frame for the useful modes and thus limit the power transfer to the outside, and, reversely, limit the sensitivity to the outside vibrating environment of the useful modes of vibration,        the frequency plane of the seismic bodies which must have a significant difference between the frequency of the useful modes and the frequency of modes of vibration not operated for the measurement in order to limit the power transfers between such modes.        
When the resonator comprises two seismic bodies positioned side by side, the resonator is balanced in translation on one of the two useful modes, for which the seismic bodies oscillate in phase opposition along the same axis but torques are generated by the other useful mode for which the seismic bodies oscillate in phase opposition along two distant and parallel axes. Such torques transmit efforts and thus power to the outside and vice versa the displacements of the seismic bodies are opposed by the torques generated in response to the angular displacement of the sensor carrier.
When the resonator comprises four seismic bodies positioned as a square, the resonator is totally balanced for the two own useful modes, but the frequency plane thereof is not optimum, since the vibration useful modes are close to not operated modes which may create power transfers with the two useful modes.
Various solutions have been proposed for improving the performances of the sensors like, for instance, in the document FR-A-2983574.